Analog-to-digital converters are in widespread use today in electronics for consumers, industrial applications, etc. Typically, analog-to-digital converters include circuitry for receiving an analog input signal and outputting a digital value proportional to the analog input signal. This digital output value is typically in the form of either a parallel word or a serial digital bit string. There are many types of analog-to-digital conversion schemes such as voltage-to-frequency conversion, charge redistribution, delta modulation, as well as others. Typically, each of these conversion schemes has its advantages and disadvantages. One type of analog-to-digital converter that has seen increasing use is the switched capacitor sigma-delta converter.
As many analog-to-digital converters, the switched capacitor sigma-delta converter uses a digital-to-analog converter in a feedback loop and cannot be more accurate than the digital-to-analog converter. Therefore a very accurate digital-to-analog converter is required in order to achieve an accurate analog-to digital conversion. However a high resolution is not required for the digital-to-analog converter used in the feedback loop of a sigma-delta converter: The digital-to-analog resolution can be exchanged with the over-sampling ratio at the cost of a longer conversion time.
A two-level digital-to-analog converter is inherently accurate and thus is not the limiting factor for the accuracy of a sigma-delta converter. Therefore it is the standard approach in a sigma-delta analog-to-digital converter.
However, what is needed is a reduction in quantization noise, over sampling ratio and power consumption of the sigma-delta analog-to-digital converter. Such a reduction is sometime achieved with a multi-level digital-to-analog converter but with the cost of an expensive trimming or a complicated dynamic element matching technique.